Field of the Invention
The present invention is in the field of fluorescent lamps and specifically relates to an improved circuit used to supply operating power to the lamps.
Fluorescent lamps have a very high electrical resistance until the mercury vapor within them has been ionized. Ionization may be accomplished by the use of heater filaments typically located at the ends of the lamp envelope, or by application of an initial high voltage. After ionization the resistance is much less and the lamps typically have a negative resistance characteristic.
Conventional circuits for powering the fluorescent tubes use a ballast to limit the steady state current through the lamp to a value which is safe and compatible with the desired light output and life of the lamp. Typically such ballasts use an inductive reactance to limit the operating current, usually in the form of a high reactance transformer. Conventional ballasts are large, heavy, expensive, and electrically inefficient wasting a substantial percentage of the power supplied to the lamp installation. The desirability of eliminating the ballast has long been known in the art.
For example, Kriege in U.S. Pat. No. 3,778,677 issued Dec. 11, 1973, raises the electrical potential of both filaments to a high A.C. voltage (of the order of 500 volts) above the ground potential, which induces ionization inside the lamp by capacitive coupling to the ground plane. Sammis in U.S. Pat. No. 3,525,901 issued Aug. 25, 1970, applies an alternating voltage of ultrasonic frequency to facilitate starting the discharge which is then sustained by a direct current. Similarly, Miyajima in U.S. Pat. No. 3,631,317 applies a unidirectional pulse train to facilitate starting.
The present invention eliminates the need for using a ballast with conventional fluorescent lamps, through means which will be described below. Elimination of the ballast, although desirable is not the main feature of the present invention.
One embodiment of the present invention makes use of a switch located in the fluorescent tube socket and actuated by removal of the tube, not unlike the switch used by Peterson in U.S. Pat. No. 2,522,111 issued May 8, 1951. Peterson used a switch to interrupt the primary power supply, and in Peterson's invention the full primary voltage would be present between the switch contacts which are open when the fluorescent tube is removed presenting a safety hazard. In the present invention, the switch contacts are closed when the tube is removed and the safety hazard is eliminated. In the present invention, a switch is used in a feedback loop to disable the converter but not the primary supply as in Peterson. Thus, it is not the switch which is novel in the present invention, but rather the use made of it in combination with the remaining circuitry.